This phrase is synonymous with "
big hands" or "large hands". It just means you have big hands and/or long fingers (which are needed to
reach around and play a bass well).
If someone tells you that you have bass player hands it means that either 1:They are just
observant and noticed your big hands
or
2:They want your fingers inside them, and they're letting you know that they are thinking about your hands.
Girl-*Notices you have large
hands*
Girl-"
Ooh you have bass player hands."
Boy-"Yeah they're really good for
reaching far into small spaces."